VFX in Viewtiful Joe - energy meter that governs all of your time-related powers, introducing a constant element of resource management to combat and puzzle solving. (It goes beyond the scope of just one game mechanic (which is vague in and of itself anyway, I guess), but the way your entire moveset, VFX powers included, work seamlessly in both combat situations and environmental puzzle-solving scenarios is also awesome.)
The implementation of melee attacks in Resident Evil 4, 5, and 6. 6's might be the best overall, honestly.
Vanquish's single meter that controls AR (slow-mo), boosting, and melee attacks. Exciting in the same way that Viewtiful Joe's VFX is.
Dead Rising's timer. Combined with the game's excellently laid-out world and strong, intelligently spaced-out quests, it makes the game really exciting to play.
Wall jumping in Super Metroid. A cool advanced trick that the game invisibly teaches to you, is challenging yet satisfying to pull off, and lets you pull off some cool sequence breaks if you figure it out by yourself early on.
Bayonetta's dodge offset, which allows you to continue combos even while dodging around enemies.
I don't want to call it one of the best game mechanics, but since I played it recently, Bionic Commando 2009's swinging is extremely satisfying, and pretty much makes the game worth it by itself.
